1. Field of the Invention
This invention relates generally to methods and apparatus for strengthening columns or elongate tubes, and more specifically for strengthening spring reinforced catheters and other surgical access devices.
2. Discussion of the Prior Art
It is often desirable to increase the strength of elongate columns or tubes, such as tubes formed from flexible plastics. Ordinarily one would increase the wall thickness of the tube in order to provide increased strength, but in some cases there are limitations on the maximum diameter which can be tolerated. Such is the case with medical catheters which require the smallest possible outer diameter.
For such catheters, it has been found desirable to form the plastic tube over a spring thereby increasing the column strength of the catheter without sacrificing either flexibility or size. The modulus of such a combination has exceeded the sum of moduli associated with the spring and the tubing.
Spring reinforced catheters have been made from several processes all of which require the application of externally generated heat. Most commonly, the spring has been inserted into the bore of flexible polyvinylchloride tubing which is then heat shrunk so that it collapses onto the spring. This is a complex process and somewhat restricted as to the materials which can be used for the tubing.
Coextrusion has also been used to manufacture spring reinforced catheters. In this process, the spring is deployed through an extruder as the molten plastic is formed around the spring. Molding processes have also been used for relatively short lengths of tubing.
In all of these methods of manufacture, heat must be applied to the tubing in significant quantities in order to effectively melt or otherwise shrink the tubing onto the spring. In its melted state, the tubing forms a sheath over the spring which is essentially free of stress due to the applied heat. This results from the fact that any stresses in the tubing are essentially relieved by the heat. The resulting structure has a relatively low modulus. Each of these methods of manufacture requires complex machinery for coextrusion, heat shrinking or molding; in addition, the related processes are extensive and must be carefully controlled.
Particularly in a catheter construction, the tubing must meet several requirements. For the processes of the prior art, it is desirable that the material be shrinkable or at least heat formable. It is desirable that it have a high tensile strength and good flexibility. Complex catheters, requiring balloons or thermistors or other associated structures, also require that the tubing material be solvent bondable.
Polyvinylchloride, polyethylene, urethanes and nylon can all be heat shrunk, but each of these materials fail to meet one or more of the forgoing criteria.